This application claims the benefit of European Application No. 00402451.9, filed Sep. 6, 2000.
1. Field of the Invention
The present invention is directed to a method of fabricating an optical grating or multiple gratings in a planar waveguide device.
2. Description of the Related Art
Periodic grating elements have numerous applications in planar optical waveguides: for example, as Bragg reflectors they can be used for spectral filtering, add/drop multiplexing or dispersion compensation elements. They also have applications as beam deflectors, waveguide lenses, mode converters and input/output couplers.
In particular, Bragg reflectors can be used in a zero-order phasar component. For example, in the commonly owned and co-pending PCT application no. WO 99/36817, a monolithic planar waveguide device is described. In this device, multiple grating elements are placed over the parallel paths of a phasar region (also referred to as an arrayed waveguide grating (xe2x80x9cAWGxe2x80x9d) region) in which each grating respectively reflects one specific wavelength and therefore the device can act as a drop-multiplexer.
For example, FIG. 1 represents a schematic layout of a device described in WO 99/36817. Optical device 10 includes an Mxc3x97N evanescent coupler 30 and Nxc3x97O coupler 70, e.g., free space Nxc3x97N couplers having a planar arrangement of two linear waveguide arrays separated by a free space region. Mxc3x97N evanescent coupler 30 has M exterior ports 20 and N interior ports 40. Exterior ports 20 are used to access the exterior of the device 10. Interior ports 40 are individually connected to N optical paths 110 to 11N. The optical paths 110 to 11N are connected at the other end to the N interior ports 60 of Nxc3x97O evanescent coupler 70. Nxc3x97O coupler 70 also includes exterior ports 80, which access the exterior of device 10. Wavelength selecting elements 50 to 5M-1 are disposed on the N optical paths 110 to 11N. Wavelength selecting element 52, e.g., a Bragg reflector, is tuned to xcex1, element 52 is tuned to xcex2, and element 5M-1 is tuned to the Mxe2x88x921th wavelength supported by device 10. Thus, light having wavelengths xcexMxe2x88x921 enters exterior port 20M and is equally divided.
In such a device, if Bragg reflectors are used as wavelength selecting elements, sub-micron positioning precision of the Bragg reflectors (50, 52, et seq.) within the phase array is needed in order to achieve the desired optical functionality of the device.
In addition to the required positioning precision, it is also desirable to utilize a Bragg reflector having a high reflectivity. For example, consider that Bragg reflectors utilized in a device such as optical device 10 are required to reflect 99.9% of the incident light power at the design wavelength to ensure a maximum crosstalk of xe2x88x9230 dB. From the coupled-wave theory of Bragg reflectors the reflectivity is given by:
|r(xcfx89)|2=|tanh(xcexaL)|2
The reflectivity specification of 99.9% implies that
xcexaL greater than 4.15
where xcexa is the grating contra-directional mode coupling coefficient and L is the grating length. Thus, in order to achieve a xcexaxc2x7L value of 4.15, either the grating length needs to be large, or the mode coupling coefficient needs to be large.
Thus, what is needed is a straightforward method of fabricating a grating in a planar waveguide device in a high precision manner. Also, what is needed is a straightforward method of fabricating a grating in a planar waveguide device to increase the coupling coefficient xcexa thereby allowing for a reduced grating length. In view of the foregoing, according to an embodiment of the present invention, a method of fabricating a grating in a planar waveguide device comprises providing a substrate material that includes a substrate layer, a first core layer, a second core layer, and a first photoresist layer. An exposure of a grating and a plurality of alignment marks is formed onto the substrate material. The second core layer is etched to form the grating in the second core layer. A second photoresist layer is deposited on the substrate material that remains after the first etching. An exposure of a waveguide pattern is formed in the first core layer. The first core layer is etched to define a first waveguide in the first core layer, where the first waveguide includes a first portion having the surface grating.
According to another embodiment of the present invention, a method of fabricating a grating in a planar waveguide device comprises providing a substrate material that includes a substrate layer, a core layer, and a first photoresist layer. A first photo-mask that includes a plurality of alignment marks is disposed between the first photoresist and a light source. An exposure of the first photo-mask is performed and the alignment marks are etched into the core layer. A grating is written into the core layer by a photosensitive effect. A second photoresist layer is deposited on the substrate material and an exposure of a waveguide pattern is formed in the core layer. The core layer is etched to define a first waveguide in the core layer, where the first waveguide includes a first portion having the surface grating.
Further features of the invention form the subject matter of the claims and will be explained in more detail, in conjunction with further advantages of the invention, with reference to exemplary embodiments.